Cerebro-Spinal Fluid
CSF

Formation:Clinical and experimental
observation has established that the cerebrospinal fluid (CSF) is
mainly formed by secretion from the choroid plexusesof the cerebral ventricles.

Circulation :That formed by the plexuses of the
lateral ventricle passes through the interventricular
formina into thethird
ventricle. Since the fluid flows through the
cerebral aqueductinto thefourth
ventricle , which it leaves by the median and to
lateral foramina (of Magendie and Luschka)of the 4th ventricle to reach the subarachnoid space covering
the cerebrum and also the spinal cord.

The subarachnoid space
(SAS), which lies between the arachnoid membrane externally
and the pia mater internally, carries the flow from the
cerebral ventricles to its points of absorption. The inner
surface of the arachnoid and the outer surface of the pia are
covered with flattened mesothelial cells; these also cover the
numerous trabeculae, which bridge the SAS, and the nerves and
blood vessels which pass across it. Cross section of brain
covering showing relation between layers

The SAS is deepest at the base of the
brain; its expansions constitute the various cisterns,
the largest of which is the cerebello-medullary cistern or cisterna
magna which lies between the cerebellum and medulla
and extends downwards below the foramen magnum behind the spinal
cord.

Absorption : The SAS extends superficially over
the whole surface of the brain and spinal cord. Every blood
vessels entering or leaving the nervous system must pass across
it. In so doing, it carries with it into the nervous system a
sleeve of arachnoid immediately surrounding the vessel and a
sleeve of pia mater more externally. Between the two lies an
extension of the SAS, known as the perivascular or Virchow-
Robin spacewhich subdivides with
each division of the vessel to terminate where the pia mater and
arachnoid become continuos.

Probably, products of
metabolism and cell - containing inflamatory exudates pass from
the perivascular spaces to enter the CSF in the SAS, but must
cross the tight junction of the capillary endothelial cells which
largely constitute the blood/ CSF and blood/brain
barriers in order to do so. These barriers may be
damaged by inflammation and other processes and it is in the
perivascular spaces that cuffs of inflammatory cells are found in
inflammatory disorders of the NS.

Much work involving clinical observation
in man and experimental studies in animals using studies of
spinal drainage of isotopic exchange, of ventriculo-cisternal
perfusion, Manometric infusion techniques, and radiographic
contrast studies, most recently employing metrizamide washout to
measure CSF bulk flow as well as biochemical investigation upon
the composition of ventricular, cisternal, and spinal samples of
fluid have shown that the fluid is largely formed by active
secretion and transport via the choroid plexuses. Vesicular
transport is probably also involved. The total volume of the
fluid at any one time probably varies between 70-120
ml in different subjects and its rate of
formation is about 0.35 ml per min.

It is thus replaced several times each
day. There is also known to be a constant process of dialysis
with exchange of various chemical constituents between the CSF
and the Blood across the ventricular empendyma, the
perivascular spaces and the arachnoid membrane at all levels.
Large molecules fail to enter the CSF from the blood because of
the interposition of the vascular endothelium (the blood CSF
barrier) but there is a rapid exchange of small molecular weight
substances between the CSF and the extracellular fluid of the
brain and cord.

The fluid acts in some respect as a
"sink" preventing the extracellular fluid of the brain
from achieving a true equilibrium with the blood plasma. The
composition of the ventricular CSF is very different from that of
cisternal and spinal fluid, indicating that some components are
added to the fluid across the spinal arachnoid.

After bathing the surface of the spinal
cord and the base of the brain, CSF passes upward over the
convexity of the hemispheres to be absorbed into the intracranial
venous sinuses. Studies of bulk flow have shown that absorption
place through the microscopic arachnoid villi,
which are minute projections of the SAS into the lumen of the
sinus. Electron microscopic studies demonstrated vacuoles within
the cells of the villi which suggested that there is a dynamic
system of transcellular channels or pores which allow the bulk
outflow of CSF across the mesothelial barrier.